The process of rolling threads on a metal shank consists essentially of placing the shank in the path of the relative movement of two coacting dies which compress and roll the shank between their striated facing surfaces to impress a helical thread form into the surface of the shank as the two dies pass each other with decreasing clearance between their faces. The surfaces of the dies may be flat and the path of their relative movement linear and reciprocating, or the surfaces of the coacting dies may be arcuate and their relative movement circular and continuous.
In either case, the facing surfaces of the dies are striated with alternate lands and grooves having a cross-sectional configuration complementary to the thread form desired on the rolled shank, and the lands and grooves are inclined at the desired lead or helix angle to the direction of relative movement of the two dies, the leads of the two dies being of equal but opposite inclination.
Coactive surfaces long enough in the direction of relative movement to cold forge the surface of the shank into a helical thread will typically have multiple thread starts where successive lands emerge from the edge of the die one shank circumference distant from one other along that edge. The emerging die land at that point is weak and sharp, a condition which has typically been relieved in part by chamfering the edge of the die along which the thread-forming lands emerge.
While this measure eliminates the knife-edge of the emerging land for the protection of those who handle the dies, it has not cured the lateral weakness of the emerging land, with the result that die failures regularly occur by the chipping out of the emerging end of the die land, and frequently also result in the chipping out of the edge of the die block, sometimes to surprising depths. This mode of failure occurs in high speed steel as well as in cemented carbide die materials and can severely limit the life of the die by destroying its ability to produce threads to required specifications.